Loudspeakers

ABSTRACT

A loudspeaker having a closed cabinet in the form of a right, regular prism with a cross-sectional area of a regular polygon having at least six corners. A bass loudspeaker element is positioned in one of the end walls of the cabinet and at least three treble loudspeaker elements are positioned in each of adjoining cabinet side walls adjacent that end wall in which the bass element is mounted, the angle between the normals to the two remote cabinet side walls in which treble elements are mounted, being at most 120*.

United States Patent Ekdahl et a].

[54] LOUDSPEAKERS [72] Inventors: Hans Ekdahl, Lund; Sten Olofsson, Mal- 211 Appl. 110.; 57,702

1451 June 20, 1972 304,309 4/1968 Sweden ..l79/l E Primary Examiner-Kathleen H. Claffy Assistant Examiner-Horst F. Brauner Attorney-Ralph E. Bucknam, Jesse D. Reingold, Robert R. Strack and Henry A. Marzullo, Jr.

[ ABSTRACT A loudspeaker having a closed cabinet in the form of a right,

52 us. Cl. ..1s1/31 B, 179/1 E 'egulaI Prism with ma 'egular P'Y8" s1 1 1m. 01. ..Gl0k 13/00, 1-1041 1/28 having at least Six comem- A has loudspeaker element is P [58] Field f Sea h ..179/1 AT, 1 E, 1 GP, 1 GA; tioned in one of the end walls of the cabinet and at least three 181/31 B treble loudspeaker elements are positioned in each of adjoining cabinet side walls adjacent that end wall in which the bass References Cited element is mounted, the angle between the normals to the two remote cabinet side walls in which treble elements are UNITED STATES PATENTS I mounted, being at most 120. 2,602,860 7 1952 Doubt. ..l79/l E l 2 Claims, 6 Drawing Figures FOREIGN PATENTS 0R APPLICATIONS 1,199,441 12/1959 France ..179/ 1 GA 5 Y 1 I I l G3 6 FIGS P'A'TENTEflJuneo I972 3. 670, 8 42 saw 3 or 3 Lllllllllllllllllllllllll] 1|||||| llllllJllllllll e c seen-3 LOUDSPEAKERS This invention relates to a loudspeaker having a closed cabinet whose walls define a suitably large air volume, a bass loudspeaker element, at least three treble loudspeaker elements, a dividing network, and the requisite leads for connecting the loudspeaker to an amplifier.

To attain true sound reproduction with the use of a program source, some type of amplifier unit and loudspeakers, certain requirements have to be placed on these links of the sound reproducing chain. As for the program source, which may be an FM tuner, a record player, a tape recorder etc., and the amplifier unit, these requirements can be fulfilled without any great technical difiiculties. The primary requirements placed on the loudspeaker or loudspeakers are that they have a favorable frequency response and give low distortion and a diffuse sound field with a correct sound pattern for providing a true sound reproduction. In most of the loudspeaker designs hitherto suggested these requirements have been but insignificantly considered and no optimization has been brought about in order to fulfill all of these requirements to the highest possible extent.

As yet no standards have been adopted for establishing the quality of a loudspeaker. Measurements of the frequency characteristic of a loudspeaker in different directions in an anechoic sound chamber, that are now often effected, do not make it possible to appraise the properties of the loudspeaker in the environment it is to be used in, for instance an echoic room of swellings. Fully objective grounds for appraising loudspeakers cannot therefore be established due to the non-existence of an average room and also due to the listener's subjective perception of the sound reproducing quality, since he will compare for instance reproduced music with living music such as he remembers it.

To have good sound reproducing properties in a conventional room a loudspeaker must have as straight and uniform frequency characteristics as possible in an anechoic sound chamber. Thus, the only fully reproducible measuring procedure is the measuring made in an anechoic sound chamber, which must, however, be supplemented with listening tests under operating conditions normal to the loudspeaker.

A cabinet that shall satisfy the requirements outlined in the foregoing must be designed in such a way that its air volume and attenuation are adapted to the bass loudspeaker element and that the localization of the treble loudspeaker elements provides an angle of radiation of approximately 130. Moreover, the treble loudspeaker elements shall be placed in small surfaces not defined by too sharp edges, and for attaining good acoustic stability, i.e. avoiding vibrations and resonances in the cabinet walls, the cabinet should not have too large unbroken surfaces.

The above conditions imply that the air volume of the cabinet shall be placed rearwardly of the bass element and in not too unsymmetrical a location relative to said element. The depth of the cabinet rearwardly of the bass element should be at least twice the diameter of the said element. The relative location of the treble elements also is of the utmost importance. In all sound reproduction it is aimed at having reflections. The reflected sound shall give a diffuse sound field without roducing echoes or destroying directional information. To attain an audible echo the reflected sound must be delayed at least 30 milliseconds, if the echo is equally strong as the direct sound. A weaker echo requires a delay of at least 50 milliseconds. The distance in travel thus must not be greater than about 17 meters, which is not, however, applicable in conventional rooms.

To obtain an enjoyable sound pattern of diffuse sound the time delay should lie within the range of 220 milliseconds, i. e. a distance of travel of 0.7-7 meters for the first or second reflection.

Mounting the treble loudspeaker elements in small surfaces requires that the bass element and the treble elements are not positioned in the same surfaces. The simplest way in which to attain this is to mount the bass element horizontally with upwardly directed radiation. Due to the expansion of lower frequencies an at least semi-spherical radiation is obtained if the frequency of the bass element is limited to not more than 3 kHz. To attain the contemplated radiation of the higher frequencies the treble elements must be at least three in number and be angularly mounted in relation to each other. Since an angle of radiation of approximately 130 is desirable for the higher frequencies the angle between the main directions of radiation of the two outermost treble elements must be approximately A certain variation, however, is conceivable, but this angle must not amount to more than ap proximately Such an arrangement of the treble elements can be realized with the use of a cabinet which has a cross-sectional area in the form of a regular polygon with at least six corners. Other cross-sectional configurations are certainly conceivable but these imply the disadvantages of a poorer acoustic stability.

The object of the present invention therefore is to provide a loudspeaker having relatively simple driving elements, said loudspeaker involving great advantages in sound reproducing respect and having substantially the following sound reproducing properties:

Acoustic balance between direct and reflected sound with a natural sound pattern in the entire room.

Balanced treble distribution which results in an optimum difiusion of the total sound field.

Acoustic stability owing to small cabinet surfaces and volume arrangement to prevent vibrations and resonances.

Symmetrical cabinet configuration with optimum acoustic adaptation of the relative location of the driving elements.

These properties are realized in a loudspeaker according to the invention in that the cabinet has the form of a right regular prism with a cross-sectional area in the form of a regular polygon having at least six comers, that the bass element is mounted substantially centrally in one cabinet end wall, and that the treble elements are so mounted in each of adjoining cabinet side walls adjacent that end wall in which the bass element is mounted that the angle between the normals to the two most remote side walls in which treble elements are mounted, is at most 120. From the foregoing, it will be appreciated by the artisan that the oppositely disposed end walls and the plurality of side walls which are connected to one another and to the end walls so as to define a closed cabinet, are dimensioned such that the cabinet is in the form of a right, regular prism of at least six sides each constituted by a corresponding side wall. Thus, the cross-section of the cabinet taken perpendicular to its longitudinal or generation axis will be a regular polygon of a number of sides equal to the number of side walls, and the cross-section of the cabinet taken parallel to its longitudinal axis will be rectangular.

The invention will be more fully described in the following with reference to the accompanying drawings in which:

FIG. 1 is a polar diagram showing the desirable directional characteristics of the treble elements of the loudspeaker;

FIG. 2 is an example of how a loudspeaker according to the invention is located;

FIGS. 3 and 4 are a diagrammatic matic side view, respectively, of an speaker;

plan view and a diagramembodiment of the loud- FIG. 5 is a sound pressure curve measured at a short distance from the respective loudspeaker elements to eliminate the influence of the measuring chamber;

FIG. 6 is a sound pressure curve of a loudspeaker according to the invention, measured at a distance of 1 meter from the loudspeaker. I

To obtain the contemplated sound pattern the loudspeaker for higher frequencies shall have directional characteristics of a primary form according to the polar diagram in FIG. 1 where three treble elements are used. The sound field of the treble elements is thus forwardly directed with a radiation angle of approximately within 6 db. This radiation angle is fully suflicient to obtain a diffuse sound field, and an omnidirectional radiation of the higher frequencies would only give further interferences and diffraction problems around the cabinet.

FIG. 2 shows an example of how to locate a loudspeaker according to the invention in a room 1. A loudspeaker is located at a point 2 in proximity to a comer of the room. For stereophonic sound reproduction a loudspeaker is required also at a point 3 in proximity to another corner of the room. As for the loudspeaker at point 2 the radiation for the high frequencies from the treble elements is partly indicated. The requirement for a time difference between direct and reflected sound after it has been reflected once from the side walls is fulfilled within the broken line for the loudspeaker at point 2. The same applies to the loudspeaker at point 3. If the reflection against the wall opposite the loudspeaker is considered, the requirement is fulfilled, i.e. the contemplated sound pattern is theoretically obtained, within the entire part of the room where direct sound occurs. On this account and due to reflections and secondary reflections in furniture and other items there is obtained in practice a diffuse sound field of very good sound pattern in an ordinary room, except in the immediate vicinity of the loudspeakers. This area, however, nevertheless is not useful in stereophonic sound reproduction.

FIGS. 3 and 4 show a loudspeaker having a cabinet 4 in the form of a right, regular prism with a cross-sectional area in the form of a regular octagon. A has loudspeaker element 5 is mounted in the upper end wall of the cabinet 4, and three treble loudspeaker elements 6 are mounted in three of the side walls of the cabinet. The said treble elements are mounted in adjoining side walls adjacent that end wall in which the bass element 5 is mounted. By this arrangement the directional characteristics for higher frequencies reproduced in FIG. 1 can be attained. Owing to the location of the treble elements 6 it is not certain that the bass element 5 can be mounted centrally in the upper end wall as shown in FIG. 3, but it may be necessary to place it at a certain selected distance from the treble units independence upon the dividing frequency f}. The difference of travel s between the sound from the treble elements and that from the bass element, as a matter of fact, must satisfy the condition: S=fl/2')t wherein n is an integer, and x =/f,,, wherein c is the sound velocity. The difference of travel in. its turn influences the phasing-in procedure in the dividing network.

The loudspeaker shown in FIGS. 3 and 4 has been tested in two sizes with cabinets of 11 liters and 33 liters,.respectively. The designations of the driving units, which were of the Peerless make, and the cabinet sizes, designated according to FIG. 4, will appear from the following table.

Cabinet 11 liters 33 liters Bass element B 65 W0 L825 WG Treble element MT225 HFC MT225 HFC a (cm) 9.6 13.3 b (cm) 23.2 32.l h (cm) 36.3 5 l .6

The curve in H6. 5 illustrates the adaptation of cabinetdriving units for the ll-liter cabinet. It is a sound pressure curve measured at a short distance from the respective driving units.

The curve in FIG. 6 is a sound pressure curve measured in an anechoic chamber at a distance of 1 meter from the loudspeaker which has an ll-liter cabinet. The curve lies well within :3 db from 75 Hz to 20 kHz, which clearly shows the excellent frequency response of the loudspeaker. The decrease below Hz in FIG. 6 is warranted by the normal bass increase obtained by locating the loudspeaker adjacent a wall in an ordinary room and which must be compensated for by means of a bass decrease such as that shown in FIG. 6.

The distortion of the ll-liter cabinet loudspeaker tested, within broad margins, is less than 1 percent right down to 100 What we claim and desire to secure by Letters Patent is:

l. A loudspeaker which comprises a pair of oppositely disposed end walls and a plurality of side walls connected to one another and to said end walls to define a closed cabinet confining an air volume, said cabinet being in the form of a right, regular prism of at least six sides each constituted by a corresponding side wall, a bass loudspeaker element mounted substantially centrally in one of said end walls, and a plurality of treble loudspeaker elements mounted one in each of at least three of said side walls, the angle between the normals to the two remote side walls in which treble loudspeaker elements are mounted being at most said has loudspeaker element mounting end wall having an opening in front of the bass loudspeaker element, and each side wall in which a treble loudspeaker element is mounted having an opening in front of the treble loudspeaker element, said cabinet having a depth rearwardly of said bass loudspeaker element at least twice the diameter thereof.

2. A loudspeaker according to claim 1 wherein said end walls and side walls define a cabinet in the form of a right, regular prism of eight sides. 

1. A loudspeaker which comprises a pair of oppositely disposed end walls and a plurality of side walls connected to one another and to said end walls to define a closed cabinet confining an air volume, said cabinet being in the form of a right, regular prism of at least six sides each constituted by a corresponding side wall, a bass loudspeaker element mounted substantially centrally in one of said end walls, and a plurality of treble loudspeaker elements mounted one in each of at least three of said side walls, the angle between the normals to the two remote side walls in which treble loudspeaker elements are mounted being at most 120*, said bass loudspeaker element mounting end wall having an opening in front of the bass loudspeaker element, and each side wall in which a treble loudspeaker element is mounted having an opening in front of the treble loudspeaker element, said cabinet having a depth rearwardly of said bass loudspeaker element at least twice the diameter thereof.
 2. A loudspeaker according to claim 1 wherein said end walls and side walls define a cabinet in the form of a right, regular prism of eight sides. 